1. Field of the Invention
This invention relates to a sensor, and more particularly, to an improved air fuel ratio sensor which has a housing formed by cold forging and a detector element fixed in the housing by crimping or staking under hot conditions. The invention also relates to a method for making such a sensor as mentioned above.
2. Description of the Prior Art
As is well known in the art, air fuel ratio sensors are arranged to detect an air fuel ratio in a gas to be measured using a signal from a pair of electrodes provided at one end of a sensor element. The sensor element is fixedly held in a accommodation hole of a housing. Where such an air fuel ratio sensor is adopted, for example, in automotive vehicles, they are inevitably exposed to water, soil, or de-frosting fluids. Accordingly, care should be taken to corrosion resistances of members which are used to constitute an air fuel ratio sensor.
To this end, it is usual to use a ferritic stainless steel of a high corrosion resistance as a housing for airtightly fixing the sensor element therewith. The housing hitherto employed for this purpose is made of ferrite-base stainless steels of good machinability. Such steels are so hard that the housing is formed by machining.
The housing essentially requires an accommodation hole wherein a sensor element is accommodated. For the formation of the hole, a material for the housing has to be machined in large quantities, e.g. about 70% of the starting steel for the housing is usually wasted in vain by machining.
To avoid the wastage of the steel, attempts have been made wherein the housing is not formed by machining, but through cold forging.
However, where a housing is formed by forging, ferritic stainless steels of the type as used for the machining cannot be employed because of the difficulty in cold forging of the stainless steels. In short, different types of ferritic stainless steels, which have properties suitable for the cold forging, become necessary.
In the fabrication of conventional air fuel ratio sensors, it is usual to form a housing having an accommodation hole, place a sensor element in the accommodation hole, and subject the accommodated element to crimping under hot or heating conditions to fixedly hold the sensor element in the hole of the housing.
However, the housing obtained by cold forging has the problem that seawater or rainwater is prone to infiltrate from the thermally crimped portion thereof, thereby causing the housing to be corroded.